Metal oxide fabrics

ABSTRACT

The specification discloses fabrics that are composed of metal oxides such as zirconia, tantala, and other metal oxides which have not heretofore been available in the form of fabrics. The metal oxide fabrics of this invention are characterized by being relatively flexible and strong, as well as exhibiting the desirable high temperature and chemical resistance properties that are typical of materials such as zirconia and tantala. The metal oxide fabrics of the invention are useful as battery separators, fuel cell separators, in high temperature insulation, and the like.

1, :11 Unite tates atom [15] 3,663,182 Hamling 1 May 16, 1972 [54] METAL OXIDE FABRICS References Cited [72] Inventor: Bernard H. Hamling, Warwick, N.Y. UNITED STATES PATENTS [73] Assignee; Union Carbide Corporation 599,018 2/1898 Simonini ..252/492 3,190,723 6/1965 .lacobsen Flledi 1963 3,270,109 8/1966 Kelsey ..23/139 X 3,322,865 5/1967 Blaze, Jr. ..264/0.5 [2!] 717367 3,341,285 9/1967 Kelsey ..23/142 Related US. Application Data Primary Examiner-Benjamin R. Padgett [63] Continuation-impart of Ser. No. 576,840, Sept. 2, Assistant s R Hellman 1966, Pat. No. 3,385,915, which is a continuation-inpart of Ser. No. 320,843, Nov. 1, 1963, abandoned, and a continuation-in-part of Ser. No. 700,031, Jan. 24, 1968.

US. Cl ..23/355, 23/20, 23/22, 23/139, 23/140, 23/143, 23/145, 23/147, 23/183, 23/184, 23/186, 23/201, 23/202, 23/354, 23/344,

Int. Cl. ..C01g 1/00, ClOf 1/00 Field of Search ..23/139, 140, 142, 20, 22, 145,

AttorneyPaul A. Rose, Gerald R. O'Brien, Jr., William R. Morgan and Charles J. Metz 57 ABSTRACT The specification discloses fabrics that are composed of metal oxides such as zirconia, tantala, and other metal oxides which have not heretofore been available in the form of fabrics. The metal oxide fabrics of this invention are characterized by being relatively flexible and strong, as well as exhibiting the desirable high temperature and chemical resistance properties that are typical of materials such as zirconia and tantala. The metal oxide fabrics of the invention are useful as battery separators, fuel cell separators, in high temperature insulation, and the like.

10 Claims, No Drawings METAL OXIDE FABRICS This application is a continuation-in-part of copending applications Ser. Nos. 576,840 filed Sept. 2, 1966, now U.S. Pat. No. 3,385915, and 700,031 filed Jan. 24, 2968. Ser. No. 576,840 was a continuation-in-part of Ser. No. 320,843, filed Nov. 1, 1963, now abandoned.

This invention relates to metal oxide fabrics. More particularly, the invention relates to fabrics that are composed of one or more oxides of the metals beryllium, magnesium, calcium, strontium, barium, scandium, yttrium, lanthanum, cerium and other rare earth metal oxides, titanium, zirconium, hafnium, vanadium, niobium, tantalum, chromium, molybdenum, tungsten, manganese, iron, cobalt, nickel, copper, zinc, cadmium, aluminum, thorium, uranium and plutonium. The preferred fabrics of the invention are composed of oxides of one or more of the metals magnesium, titanium, zirconium, hafnium, niobium, yttrium, cerium, tantalum, tungsten, thorium, and aluminum.

By the term fabric" is meant a composition that is composed of fibers that have been interlocked. Thus, woven, knitted, braided, and the like articles are included within the definition of fabric. Also, fabric includes certain felts wherein the individual fibers have been interlocked, for example, by needle punching, after felting.

The prior art has produced fabrics out of certain metal oxides such as silica which can be produced by first melt drawing the individual fibers followed by weaving, knitting, and the like, into a fabric. Silica fabric, which is usually known as glass cloth, is relatively flexible and strong. Fabrics have also been made from thoria for use in the well known gas mantles. Thoria heating mantles have been prepared by impregnating a rayon or other cellulosic fiber sock with a thorium-containing compound, and then burning off the precursor fabric to produce the thoria heating mantle. These heating mantles were extremely brittle and could not be handled to any appreciable degree without breaking. Other metal oxides such as zirconia, alumina, and the like have been produced in the form of individual fibers by rather arduous methods. However, these fibers could not be woven, knitted or the like into fabrics because they were so very brittle. The present invention provides metal oxide fabrics such as zirconia, alumina, and tantala, which are relatively flexible and strong and which can be subjected to reasonable handling without breaking. For instance, the fabrics can be subjected to compression loads of at least about psi up to 100 psi, or more, without failure.

The metal oxide fabrics of this invention can be produced by the process that is disclosed and claimed in parent applications Ser. Nos. 576,840 filed Sept. 2, 1966, now U.S. Pat. No. 3385915 and 320,843 filed Nov. 1, 1963, now abandoned. Briefly, this process comprises the steps of l) impregnating a preformed organic fabric with one or more compounds (preferably salts or hydrolysis products of salts) of metal elements which form the metal oxides used in this invention and (2) heating the impregnated fabric under controlled conditions (which prevent ignition of the precursor fabric) and at least in part the presence of an oxidizing gas to (a) convert, i.e., pyrolyze, the organic fabric to predominantly carbon and thereafter remove the carbon as a carbon-containing gas, and (b) oxidize the metal compound or compounds to their respective metal oxides. There is thus produced a metal oxide fabric which has essentially the same physical configuration as the original polymeric fabric, although the dimensions will be reduced.

The preferred polymeric fabric for use as the precursor fabric in the process for producing the metal oxides of the invention is composed of rayon. However, other fabrics can be used such as saponified cellulose acetate, cotton, wool, silk, man-made acrylics, polyesters, and the like. The metal compound can be impregnated into the organic fabric by any of several methods. Where the element which will appear in the final metal oxide fabric has salts which are highly soluble in water, the impregnation step can be carried out by immersing the organic fabric in a concentrated aqueous solution of such salt. Preswelling the organic fabric in water or other suitable solvent prior to immersion in the aqueous solution of said metal salt is useful in many cases to facilitate impregnation of the metal compound into the organic fabric. Among the metal compounds which can be impregnated into the organic fabric by immersing said fabric in an aqueous solution, there can be mentioned aluminum nitrate, aluminum chloride, zirconyl chloride, zirconium acetate, zirconium oxalate, zirconium citrate, yttrium acetate, yttrium trichloride, uranyl chloride, nickel chloride, chromium chloride, ferric chloride, cupric chloride, magnesium chloride, tantalum oxalate, thorium chloride, niobium oxalate, titanium trichloride, and the like.

An alternate method of loading the organic fabric is to employ compounds which hydrolyze or react with water to form metal oxide products which are essentially insoluble in water. This chemical property is utilized to effect impregnation of organic fabrics with these metal oxides as described below. Suitable hydrolyzable and/or water reactive compounds include the following; vanadyl trichloride, vanadium trichloride, and vanadium tetrachloride to yield vanadium pentoxide;

niobium oxychloride, niobium bromide, and niobium chloride to yield niobium pentoxide hydrate; tantalum pentachloride, tantalum pentabromide to yield tantalum pentoxide hydrate; molybdenum pentachloride, molybdenum oxychloride, to yield a molybdenum oxide hydrate; tungsten pentachloride, tungsten hexachloride to yield tungsten trioxide hydrate (tungstic acid); and the like. The above metal halides or oxyhalides can be dissolved in an organic liquid that is immiscible with water, such as carbon tetrachloride, chloroform, carbon disulfide, diethyl ether, or benzene, to the extent of from 5 to 50 grams of metal halide or oxyhalide per milliliters of organic liquid. A rayon or other cellulosic fabric is exposed to water in order for the fabric to swell by absorption of water. While still swollen and containing the absorbed water, the fabric is contacted with the metal halide or oxyhalide by immersion in liquid or gaseous halide or oxyhalide or in an organic solvent solution of the halide or oxyhalide. As the metal compound penetrates the moist material, it reacts with the absorbed water and an oxide precipitate forms directly in the rayon or other cellulosic fabric structure. This hydrolysis reaction is normally complete in from 20 to 30 minutes.

The extent or amount of metal deposited within the organic fabric is directly a function of the amount of water absorbed in the material. Typical hydrolysis reaction are the following:

TiCl, 2H O TiO 4HC1 2TaCl 5H O Ta O IOl-lCl The amount of water absorbed in the organic fabric is readily controlled by exposing the material to air containing the desired amount of moisture. For maximum water absorption the fabric can be immersed directly in liquid water. For example, the amount of water absorbed in textile-grade viscose rayon fabric in equilibrium with moisture in air and liquid water at 75 F. is shown below:

Moisture content,

Some hydrolyzable metal compounds are liquid at normal conditions and the water-laden organic fabric can be immersed directly in the metal compound to cause the hydrolysis product to be formed in the fabric. Examples of liquid metal halides are TiCl VOCl and VCl,. However, many of the hydrolysis reactions proceed very rapidly with the evolution of heat. The resulting severe conditions may degrade or break up the organic fabric. in this event, the metal compound is preferably diluted with a non-reactive, miscible liquid to avoid such conditions. Many non-polar organic liquids, such as benzene, toluene, hexane, carbon tetrachloride, chloroform, are suitable non-reactive liquids. These organic liquids when used as diluents for the metal compounds, slow the rates of hydrolysis and help to dissipate the heat of reaction. Unreacted metal compound liquid (as well as any diluent) may be removed from between the fibers by evaporation, since they have high vapor pressures.

Other metal compounds which can be incorporated in polymeric organic fabrics by hydrolysis reaction but which are not normally liquids are best utilized when dissolved in a nonreactive liquid which is immiscible with water. Such metal compounds, for example, include TaCl NbCl, ZrCl,, UCl,,. Suitable solvents are bromoform, carbon tetrachloride, diethyl ether, and nitrobenzene.

Following impregnation of the organic fabric with metal compound or compounds from a solvent solution, it is desirable to remove excess solution from between the individual organic fibers before the solution dries in order to avoid bonding together of the fibers by caked salt. Such removal of excess solution can be effected by centrifuging, by blotting, by pressing, or by other convenient means. The impregnated organic fabric is then thoroughly dried by any convenient means such as by air drying or by heating in a stream of warm gas. It is desirable to dry the impregnated fabric rapidly (in about 1 hour or less) to prevent expulsion of metal compound from the interior of the fabric to its surface.

When a fabric containing two or more metal oxides is desired, the organic precursor fabric is impregnated with two or more salts or hydrolysis products thereof. If two or more water-soluble salts are employed, the impregnation can be carried out by a single immersion in an aqueous solution containing both salts. When two oxides are desired, one of which is impregnated into the organic material from aqueous solution and the second impregnated by hydrolysis of the corresponding metal halide or oxyhalide as taught above, a preferred method is to first impregnate with the hydrolysis product and then with the water-soluble salt.

In the next principal step in the process for producing the metal oxide fabrics of the invention, the impregnated organic fabric is heated under controlled conditions for a time sufficient to decompose the organic fabric structure and form a carbonaceous relic containing the metal compound in finely dispersed form, and concurrently and/or subsequently to eliminate carbon and convert the metal compound to metal oxide. The controlled conditions must be such as to avoid ignition of the organic fabric. Ignition generally takes the form of an uncontrolled temperature increase within the fabric rather than combustion accompanied by flame. An uncontrolled temperature increase is a rapid increase which deviates sharply from the heating pattern of the impregnated organic fabric and its environment. When pyrolysis conditions are properly controlled, the temperature increase in the impregnated organic fabric follows closely the temperature of its surroundings (atmosphere, furnace wall, and the like) even though the exact temperature of the organic fabric may fluctuate to temperatures both above and below the nominal temperature of the environment. If the organic fabric ignites or burns instead of carbonizes, the metal compound temperature rises excessively owing to its contiguous relation to the organic fabric structure. Under such circumstances it is impossible to control the temperature, and the melting point of the intermediate metal compounds formed may be exceeded or excessive crystallization and grain growth can occur. Also, the metal compound may become suspended in the pyrolysis product vapors, and are thus lost from the environment and unavailable to form the desired relic fabric. When ignition is avoided the products have smoother surfaces and are stronger owing to a more orderly consolidation of the metal compound particles.

In practice, a convenient way of determining whether or not ignition has taken place during the heating steps is to observe degree of shrinkage or consolidation of the starting organic fabric. Where ignition has not taken place, the impregnated organic fabric undergoes substantial shrinkage along its longest dimension, generally of the order of 40 to 60 percent. (Shrinkage from 10 centimeters long to 5 centimeters long is considered to be 50 percent shrinkage.) The final product is strong and microcrystalline, and is relatively flexible. On the other hand, where undesirable ignition takes place during the heating steps, the degree of shrinkage is considerably less, and the resulting product tends to be crystalline rather than microcrystalline and it is brittle and of low strength. lf ignition takes place toward the end of the carbonization-oxidation step, the degree of shrinkage may still be substantial but the physical properties of the product will be much less desirable. In general, the degree of shrinkage is inversely proportional to the loading of metal compounds to the organic fabric. It has been found desirable to adjust process conditions to obtain maximum shrinkage for the particular metal compound loading in the impregnated organic fabric.

The shrinkage is beneficial for many end-use applications. For instance, the fabrics of the invention can be made with a much finer pore size than can be obtained by weaving individual fibers. Smaller pore size is beneficial in thermal insulation and to promote wicking of electrolytes when the fabrics are used as battery separators and in fuel cells. Also, with finer pore size, higher bubble pressure can be obtained, which is beneficial in fuel cells.

In practice, it has been found that ignition can be avoided by use of controlled reaction conditions, particularly conditions which avoid sharp changes in temperature, atmosphere composition and the like. Sharp changes in conditions tend to precipitate the uncontrolled temperature increases within the impregnated organic fabric which constitute ignition as hereinabove defined.

As an example of such controlled conditions, cellulosic fabrics impregnated with metal salts are heated to a temperature between about 350 C. and 900 C. at a rate of not more than C. per hour in an atmosphere containing between 5 and about 25 volume per cent oxidizing gas. Obviously, the heating step should not be set so that the decomposition or melting point of the metal oxide fabric product is exceeded. Typical decomposition and melting points of metal oxides that are used in the fabrics of the invention are as follows:

TABLE I Decomposition (D) Decomposition (D) includes other rare earth oxides of the formula M,O;,, wherein M is a rare earth metal.

By the time the impregnated fabric has been heated to 350 C. or above, under the above described conditions, a major portion of the cellulosic fabric will have been pyrolyzed to carbon and the carbon removed as a carbon-containing gas through reaction with the oxidizing gas, and a major portion of the metal element in the impregnated fabric will have been oxidized to the metal oxide form. The impregnated fabric should, however, be maintained at a temperature between 350 C. and 900 C., or higher, in an oxidizing gas-containing atmosphere until substantially all of the impregnated fabric has been carbonized and volatilized, and substantially all of the metal has been oxidized to the metal oxide form.

The preferred oxidizing gas is oxygen, and the oxidizing atmosphere in most cases can be air.

A preferred process for producing the metal oxide fabrics of this invention comprises (1) impregnating a rayon fabric as described hereinabove with a metal compound whose oxide has a melting or decomposition temperature above 800 C., and (2) heating the impregnated fabric to a temperature between about 400 C. and 800 C. at a rate between C. per hour and 100 C. per hour in air, and thereafter maintaining the fabric at a temperature between 400 C. and l,00O C. in air until substantially all of the rayon fabric has been carbonized and volatilized and substantially all of the metal has been oxidized to the metal oxide. Preferred finishing temperatures for Ta O are about 600 C., for zirconia stabilized with yttria are about l,000-l ,200 C., and for alumina are about 800 C.

While certain important aspects of the process for producing the metal oxide fabrics of the invention have been discussed here in general, a full disclosure of the process for producing metal oxide fabrics is found in applicant's parent applications Ser. Nos. 320,843, filed Nov. 1, 1963, now abandoned, and 576,840, filed Sept. 2, 1966, now US. Pat. No. 3385915, and in Belgium Pat. No. 697,315, filed Apr. 20, 1967, which was made available to the public in late October, 1967. The disclosures of said parent applications and said Belgian Patent are incorporated herein by reference.

The metal oxide fabrics of this invention are composed of one or more of the metal oxides listed in Table I hereinabove, and they normally contain only traces of other metal oxides or impurities. However, it is within the scope of the invention to include other materials in the metal oxide fabrics which can normally be expected to be included in a metal oxide composition. Such other materials include stabilizers, which are in many cases also metal oxides that are included within the invention. As an example, it is preferred to employ a metal oxide stabilizer in zirconia fabrics in order to prevent catastrophic crystallographic phase changes that will occur when unstabilized zirconia is heated above 600 C. The stabilizers that are normally used with zirconia include yttria, ceria, mixtures of rare earth metal oxides, calcia, magnesia, and the like. The preparation of zirconia fabrics that are stabilized in the tetragonal crystallographic form is disclosed and claimed in applicant's copending application Ser. No. 700,031, filed Jan. 24, 1968, the disclosure of which is incorporated herein by reference.

Although the metal oxide fabrics of this invention are substantially amorphous (microcrystalline), their application is not limited to conditions and environments wherein the metal oxide fabrics remain entirely in the microcrystalline state. The desirable mechanical properties of the metal oxide fabrics of the invention are retained to a large degree even after some crystallization of the metal oxides has occurred. The mechanical properties of the fabrics are seriously impaired only after the size of the crystallites is such that crystalline grains can be detected by conventional optical microscopic techniques, that is, after the microcrystalline structure is replaced by relatively large crystalline areas. For instance, mechanical properties become impaired when the size of the crystallites exceeds approximately one-tenth of the diameter of the individual fibers in the fabric.

The metal oxide fabrics of this invention which are most preferred are those which are composed almost entirely of one or more of the metal oxides of Table l hereinabove, and which contain only trace or insignificant amounts of other metal oxides or impurities.

The metal oxides listed in Table I are the oxides usually formed when an organic fabric impregnated with compounds of one or more metals is pyrolyzed and oxidized according to the teachings herein, and represent the highest normal oxidation state of the metal. However, the invention includes metal oxide fabrics comprising a total of at least percent by weight of lower metal oxides (including oxides of non-integral valence states) of the elements of Table l, for example FeO, NiO, W0 Nb O V 0 Cu O, and U0 The lower metal oxides can be obtained by partial reduction of the higher oxides, or they can be obtained as intermediate oxidation states during the oxidation step in the process for producing the metal oxide fabrics.

A particularly important class of metal oxide fabrics are those comprising a total of at least 80 weight percent of one or more of the metal oxides having melting points above 1,728 C. (the melting point of silica), and in particular products composed principally of one'or more of the oxides A1 0 BeO, CaO, CeO HfO Ta O, U0 ThO MgO, TiO, and ZrO which possess exceptional chemical inertness and strength at high temperatures. These metal oxides in dense, sintered forms comprise an important class of commercial refractory oxides. Fabrics of these refractory metal oxides can be used to make good heat shields and ablation reinforcement materials, and can be employed for reinforcing plastics for use at relatively low temperatures, and metals and porcelains and other ceramic bodies for use at high temperatures. These refractory metal oxide fabrics make excellent filters for corrosive gases and liquids at elevated temperatures. Applications include filtering molten metals, molten salts, super heated air and furnace exhaust gases. In addition to filtering applications, the metal oxide fabrics of this invention are also useful in the preparation of structural elements and as thermal insulation elements for use at high temperatures and in corrosive atmospheres, and the products of this invention which contain cerium oxide and zirconium oxide are particularly advantageous for use in contact with corrosive or alkaline liquids such as battery electrolytes. Zirconium oxide fabrics are especially useful because of their low thermal conductivity and because of their extreme high temperature stability even in alkaline environments.

Fabrication of ceramic blocks, linings, crucibles and more complex shapes by prior art methods are often difficult because of the problems attendant to fusing and sintering them into a cohesive mass. Refractory bodies frequently suffer self-destruction if temperatures are varied abruptly due to their poor resistance to thermal shock. Starting with the refractory fabrics of the invention, shaping and sintering to cohesive, complex shapes can be achieved readily. Thinwalled tubes may be made by this method and applications include furnace liners, pyrometer protection tubes and piping for liquid metals.

The metal oxides of Table l which have relatively low melting or decomposition temperatures, for example, the oxides of vanadium, molybdenum, tungsten, manganese, cobalt, nickel, copper, zinc and cadmium are known to be useful as catalysts in a variety of processes. The fabric form of these oxides, as produced by the process of this invention, possesses the same catalytic properties and catalytic uses as the conventional non-fiber form of these oxides. A particularly useful form of catalyst, for example, is a fabric of this invention which comprises from 80 to 98 percent of one of the refractory metal oxides and from 20 to 2 percent of one of the lower melting point catalytically active metal oxides.

Fabrics constructed from micron-size fibers containing uranium oxide or plutonium oxide are useful for fuel elements in nuclear reactors, especially those designed for radiationchemical processing by the use of the Kinetic energy of fission recoil particles. The fabrics can be composed of only uranium oxide or plutonium oxide, or as a mixture with other metal oxides of Table I, such as BeO, A1 0 and ZrO For use as fuel elements in other types of nuclear reactors, for research and testing purposes, vehicle propulsion, or electrical power generation, it may be desirable to coat these fabrics with an impervious material to prevent the escape of radioactive fission products into the reactor coolant stream. For use at high temperatures or in non-oxidizing atmospheres, uranium dioxide, U is the preferred form and is obtained by reducing the uranium trioxide, U0 with hydrogen gas at 500-600 C. A preferred fabric of this invention for use as a nuclear fuel element under oxidizing conditions is one which comprises from 20 to 60 weight percent uranium trioxide and the balance (80 to 40 weight percent) aluminum or zirconium oxide.

Metal oxide fabrics of this invention containing uranium dioxide are generally useful as nuclear fuel elements.

The metal oxide fabrics of this invention substantially retain the characteristic physical textile attributes of the precursor organic fabric, although they are substantially reduced in dimension. The metal oxide fabrics are characterized by relatively high tensile strength and flexibility, and they possess the property of being able to withstand repeated folding without failure.

The following examples illustrate the production of the metal oxide fabrics of this invention. Throughout the specification, including the illustrative examples, the temperature given are furnace temperatures. The actual temperature of the fabric undergoing processing by the method of this invention, as discussed hereinabove, may differ somewhat from the furnace temperature.

EXAMPLE 1 This example illustrates the impregnation and conversion of a woven cloth into a woven metal oxide (alumina) cloth. A one-square foot piece of rayon cloth weighing 41.4 gm., made up of textile yarn (3,300 denier/ 1,100 filaments/3 ply), and having 17 yarns/inch warp and 8% yarns/inch fill, was preswelled by immersion in water for one hour. After thorough blotting, the cloth contained 0.81 gram water per gm. cloth. The water-swollen cloth was immersed in aqueous aluminum chloride solution for a period of 65 hours. Solution concentration at the end of this period was 2.5 molar AlCl After centrifuging excess solution from the cloth, it was dried rapidly in recirculating air heated to 50 C. The dried cloth contained 0.69 gm. salt/gm. of rayon and was converted to alumina by heating in air gradually to 400 C. in a period of 48 hours. Traces of carbon were removed from the cloth by heating in air at 800 C. for hours.

The white alumina cloth so produced had a high luster and felt weighed 19 ounches/yd and had a bulk density of 7.9 pounds/ft. The felt possessed a high degree of flexibility and a breaking strength of 2 pounds/inch of width.

EXAMPLE 3 A one-square foot piece of rayon cloth weighing 56.2 gms., made up of high-tenacity rayon yarn in a basket weave and having a yarn count of 19 yams/inch in both warp and fill directions, was pre-swollen in water and immersed in a 2.86 molar zirconyl chloride aqueous solution at 22 C. for a period of 46 hours. The solution concentration at the end of the immersion period was 2.55 molar. The cloth was centrifuged and dried rapidly in recirculating air heated at 50 C. The dried cloth contained 0.96 grams of zirconyl chloride salt/gram rayon. The salt-loaded rayon cloth was converted to zirconium oxide cloth by heating in air gradually to 500 C. in a period of 30 hours (about 16 C. per hour). The cloth was maintained at 500 C. in air for 6 additional hours to remove remaining traces of carbon. The tan-colored zirconium oxide cloth weighed 14.2 grams and had a yarn count of 48 yarns/inch. The cloth was flexible and had a breaking strength of 6 pounds per inch of width.

EXAMPLE 4 A series of zirconia cloths containing varying proportions of yttria were produced by the relic process. The initial cloth substrate was square weave, textile rayon. The rayon was preswollen in IN hydrochloric acid, then rinsed in water. The cloth was impregnated by immersing it for 18 hours in 2.5 molar aqueous ZrOCl containing varying quantities of YCl The impregnated cloth was centrifuged three times at 4,0004,600 rpm in an l l-inch diameter basket for a total of 20 minutes in order to remove excess salt solution. The cloth was then heated in a forced air oven. The initial temperature was 25 C., and the temperature was gradually increased over a period of 24 hours to a final temperature of 650 C. Thereafter, the cloth was fired as indicated in Table 11 below.

Table 11, below, displays the YCl content of the impregnating solution, the mole percent 1 0;, in the ZrO cloth, and the crystallographic phases in the cloth after heat treating in air. The crystallographic phases were determined at room temperature by X-ray diffraction analysis. In Table 11, M, T and C denote the monoclinic, tetragonal, and cubic phases, respectively, present in the Zr0 cloths.

TABLE II Crystallographic phases of Zt'Oz/YzOg cloth YCl: Mole Crystal structure altorcontent percent Cloth 01' solution, Yz03 1 hr. at 1 hr. at 1 hr. at 30 hrs. at No. gin/liter in cloth 1,000 0. 1,200 C. 1,400 C. 1,400 O.

0 0 93% M, 7% 'I 100% M 100% M 100% M 3. 6 1. 20 90% M, 10% T 1.00% M 100% M 100% M 7. 2 2. 08 9% M, 01% '1 43% M, 57% T 96% M, 4% T 100% M 13.9 2. 73 100% T 1 0% T 100% T 100% T 26. 2 5. 52 100% C 100% C 100% C 100% C 37. 4 7.10 100% C 100% C 100% C 100% C 47. 8 l). 57 100% C 100% C- 100% C 100% C was very flexible. The alumina cloth weighed 7.1 grams and 60 EXAMPLE 5 had shrunk to an area of0. 16 square foot. A pull of 3.6 pounds per inch of width was required to tear the cloth. Spectrographic analysis of the alumina cloth showed it to contain 99.5 percent A1 0 The major trace impurity was Zn, which was contained in the starting rayon cloth and could have been eliminated by washing the rayon with dilute hydrochloric acid. X-ray powder diffraction analysis showed the cloth to be essentially amorphous alumina. A trace of poorly crystallized gamma-alumina was present as indicated by a broad diffraction band at a 2-theta diffraction angle of 44.46.

EXAMPLE 2 A piece of rayon felt, weighing 30 ounces/yd, was impregnated with aluminum chloride salt and converted to alumina felt by the method described in Example 1. The alumina A free-standing zirconia thermal insulation heat shield was fabricated. The heat shield assembly consisted of two halfcylinders measuring 4.3 inches internal diameter by 7.0 inches tall, with top and bottom circular plates. The wall thickness of the cylinder varied from 0.150 to 0.200 inch. The heat shield was produced by the following procedure:

Thirty mil satin weave zirconia cloth containing 2.2 mole percent yttria stabilizer was produced from rayon by the relic process by a procedure analogous to that described in Example 4. The cloth was fired to a final temperature of l,000 C. The zirconia cloth was then impregnated with a zirconia cement and wound around a cylinder to give a six-layer laminate. The zirconia cement is a mixture of parts by weight of an aqueous solution of basic zirconyl chloride i.e., ZrO (OH)ClnH O, and yttrium trichloride (Sp. Gr. of the solution was 1.65 at about room temperature) in proportions to yield 4 weight percent 2.2 mole percent)yttria in zirconia when fired, and 100 parts by weight of a zirconia powder containing 4 weight percent yttria. The powder was prepared by flash decomposition at 600-800 C. of an aqueous solution of zirconyl chloride and yttrium trichloride, followed by dry ball milling of the decomposition product for 20 hours and passing the powder through a 400 mesh screen. The weight of the cloth and cement was 340 and 220 grams, respectively. The laminate was then fired to a temperature of 1,000 C., over a period of 4 hours.

The heat shield was placed inside a tungsten-heated vacuum furnace. The furnace was cycled through 20 rapid heating and cooling cycles ranging from room temperature to 850 C. to 2,000 C. with no apparent damage to the heat shield other than a slight amount of warping. Unstabilized zirconia would have shattered during this test, probably during the first cycle.

EXAMPLE 6 A knitted rayon sock impregnated with a thorium compound, an article of commerce for use as an incandescent mantle in a Coleman gas lantern, was converted to the oxide by heating in air to 400 C. in a period of 7 hours and holding at 400 C. in an oxygen stream for an additional 24 hours. After this treatment the rayon was completely decomposed and the carbon volatilized from the knitted sock. The resulting sock was composed of white, lustrous thoria fibers of an amorphous structure. The sock was extremely flexible, similar to the starting rayon knitted sock. When viewed under the microscope at a magnification of 50, the thoria fibers appeared transparent to light (similar to window glass).

A second, and identical, knitted rayon sock impregnated with a thorium compound was burned off to the oxide form rapidly with a gas flame as is normally done for use as an incandescent gas mantle. The resulting thoria sock contained white fibers with a dull luster. The fibers also were very brittle and had little strength. X-ray difiraction pattern showed the thoria to be fairly well crystallized. The brittleness and low strength of thoria fibers made by burning off the rayon is attributed to crystallization and a large number of voids remaining in the fiber structure. In contrast, useful properties, such as high strength and flexibility, are developed by slow conversion to the amorphous metal oxide form at a low temperature by the process of this invention, as demonstrated in the preceding paragraph of this example.

EXAMPLE 7 Tantalum Oxide Cloth Cloth used was woven in a S-harness satin weave construction in both fill and warp directions, using bright regular viscose rayon yarn, 1,100 denier/480 filaments/2 ply. Weight of the rayon cloth was 17 oz/yd. The cloth was dry-cleaned, prior to salt-impregnation, to remove finishes which would interfere with imbibition of salt into the fibers. A 32 X 36 inch piece of this cloth, weighing 398 grams, was immersed at 75 F. in an aqueous solution of tantalum oxalate having a sp. gr. of 1.43. After 3 hours, it was removed from the solution and passed through 10 inch diameter rubber-coated padder rolls at 8 tons pressure and a speed of 8 ft/min. to eliminate excess solution from the cloth. The cloth was dried thoroughly in air.

at 75 F passed back through the padder rolls under 8 ton pressure in order to remove some stiffness in the cloth. It was next quickly; i.e., all the cloth at the same time, immersed in 58% NI LOH (aqueous) solution for minutes. The cloth was next washed thoroughly with demineralized water until a neutral pH was reached.

The ammonia and water wash treatment described above is used to precipitate the tantalum in the rayon fibers as the insoluble hydroxide and to remove the oxalate cation, soluble in water as ammonium oxalate, from the fibers. Removal of oxalate cation has been found to be beneficial for the subsequent oxidation pyrolysis step and gives improved strength and flexibility to the cloth.

The wet cloth was next dried thoroughly in air at 75 F., and again passed through the padder rolls at 8 ton pressure to eliminate the stiffness in the cloth.

The tantalum-loaded rayon cloth was converted to the T3 0, form by placing it in a forced-air circulated oven on an aluminum-mesh screen and raising the temperature of the oven at an average rate of 30 C./hr. to 400 C., with a temperature hold period of 4 hours in the region 225-250 C. As a finishing operation to remove residual carbon and other volatile impurities, the Ta O cloth was held at 600 C. in air for a period of 24 hours.

The product Ta O cloth weighed 74 grams and measured 11.5 X 13.5 X .025 inch thick. The cloth was uniformly white and showed high luster. Strength of the cloth was 6.0 lb/in. width. The fibers in the cloth are non-crystalline (amorphous); i.e., crystallites are too small to be identified by X-ray diffraction. Chemical analysis proved the cloth to be composed of Ta O,, 98.99 wt. SO 0.32 wt. C, 0.10 wt. Cl, 0.05 wt. Moisture, 0.15 wt. The product cloth has a bubble pressure of 1.0 to 1.5 psi when saturated with water (or other aqueous electrolyte). The product cloth has exceptionally good resistance to attack by -100 percent phosphoric and sulfuric acid at temperatures of 200-400 F., and is an efficient, stable electrode separator and electrolyte matrix structure in phosphoric and sulfuric acid fuel cells.

EXAMPLE 8 Niobium Oxide Cloth Niobium oxide (Nb,O cloth was made using the same rayon cloth and procedure described in the tantalum oxide cloth example.

A piece of 17 oz/yd satin weave rayon cloth, measuring 18 X 24 inch and weighing 149 grams was immersed for 3 hours at 72 F. in an aqueous niobium oxalate solution having a specific gravity of 1.198. The niobium oxalate solution is an article of commerce. After impregnation, the salt-loaded cloth was treated in an identical manner as the tantalum oxide cloth in the previous example, except that it was given a finish heattreatrnent at 500 C. in air for 18 hours.

The product niobium oxide cloth had dimensions of 7.0 X 9.75 X 0.26 inch thick, and weighed 15 grams. The cloth was uniformly white, very flexible and had 2-3 lb/in. width breaking strength. The Nb O content of the cloth was 98.0 weight percent. The oxide fibers were non-crystalline as determined by X-ray diffraction analysis.

EXAMPLE 9 Titanium Oxide Felt The precursor felt used for producing titanium dioxide (TiO felt was a mechanically-interlocked (ncedled) fiber mat using 1.5 denier regular bright viscose rayon staple fibers 1 9/16 inches long. The rayon felt had the following specifications: weight, 14 oz/yd; thickness, 0.063 inch.

A 12 X 13 inch piece of rayon felt weighing 40.4 grams was immersed at 72 F. in an aqueous solution of titanium trichloride having a specific gravity of 1.222 for a period of 3 hours. To remove excess salt solution, the felt was blotted in paper towels and centrifuged in an 11 inch diameter bowl at 4,000 rpm for 5 minutes. It was next dried, and contacted with 58 percent ammonium hydroxide for 10 minutes, followed by washing thoroughly with demineralized water, until a neutral pH was reached.

After drying the wet felt at room conditions, the loaded felt was converted to the oxide state by heating it in an air-recirculated oven at a rate of 20 C./hr. to 350 C., with a hold period of 5 hours at 350 C. It was next raised to and held at 600 C. in air for 2 hours.

The product TiO felt had dimensions of 5.0 X 6.1 inches and weighed 6.2 grams. The thickness of the felt was 0.060-0.065 inch. The uniformly white felt product was flexible it could be rolled into a spiral around a V4 inch diameter rod, and can be subjected to compression to 0.030 inch, without damage to the felt.

What is claimed is:

'25 pounds per square inch, and said fibers being composed of metal oxide in microcrystalline form, the individual crystallites of which have a size not exceeding approximately onetenth of the diameter of the individual fibers of said fabric.

2. The metal oxide fabric of claim 1 wherein said metal oxide fabric is composed of zirconia.

3. The metal oxide fabric of claim 1 wherein said metal oxide fabric is composed of alumina.

4. The metal oxide fabric of claim 1 wherein said metal oxide fabric is composed of tantala.

5. The zirconia fabric of claim 2 wherein said fabric is in the form of a woven fabric.

6. The zirconia fabric of claim 2 wherein said fabric is in the form of a non-woven fabric having at least some of the individual fibers interlocked.

7. The zirconia fabric of claim 6 wherein said fabric is in the form of a felt.

8. The zirconia fabric of claim 2 wherein the zirconia in said fabric is in the cubic crystallographic phase.

9. The zirconia fabric of claim 8 wherein said zirconia contains from about 5.5 2 to about 9.57 mole per cent yttria, based upon moles of yttria plus zirconia.

10. A thoria fabric composed of interlocked thoria fibers, said fabric being capable of withstanding compression loads of at least about 25 pounds per square inch, and said fibers being composed of thoria in the microcrystalline form, the individual crystallites of which have a size not exceeding onetenth of the diameter of the individual fibers of said fabric. 

2. The metal oxide fabric of claim 1 wherein said metal oxide fabric is composed of zirconia.
 3. The metal oxide fabric of claim 1 wherein said metal oxide fabric is composed of alumina.
 4. The metal oxide fabric of claim 1 wherein said metal oxide fabric is composed of tantala.
 5. The zirconia fabric of claim 2 wherein said fabric is in the form of a woven fabric.
 6. The zirconia fabric of claim 2 wherein said fabric is in the form of a non-woven fabric having at least some of the individual fibers interlocked.
 7. The zirconia fabric of claim 6 wherein said fabric is in the form of a felt.
 8. The zirconia fabric of claim 2 wherein the zirconia in said fabric is in the cubic crystallographic phase.
 9. The zirconia fabric of claim 8 wherein said zirconia contains from about 5.52 to about 9.57 mole per cent yttria, based upon moles of yttria plus zirconia.
 10. A thoria fabric composed of interlocked thoria fibers, said fabric being capable of withstanding compression loads of at least about 25 pounds per square inch, and said fibers being composed of thoria in the microcrystalline form, the individual crystallites of which have a size not exceeding one-tenth of the diameter of the individual fibers of said fabric. 